The white of the page is not a solid white, but full of variance.
Also, there are many tiny specs of dust and things that show up as grey specs on the image.

Prepping a black-and-white line drawing is easy; I just use color curves to set the mostly-white areas to pure white, doing the same with black, and tweaking the curve how I want to make my black lines sharper or smoother and cutting out those grey specs.

But I've been trying to work out how to add a second level of detail in these drawings. This comes in the form of a light blue. This light blue can be used for so many great things, like points to help line up an image that was drawn on multiple sheets or to represent lines that will becomes shadows or highlights that I don't want to show up in the final image.

Previously I had been scanning my pages as greyscale images to completely wipe out extra color variations that I didn't want. If I do that with the blue lines they turn grey and they get eroded when I adjust the color curves. If I scan it as full color I now have subtle color variants that throw things off, and adjusting the color curves still erodes those blue lines even worse.
If I try adjusting the color curves by color channel I turn the whole image to a particular tint.

How can I adjust the colors of an image to turn anything mostly white into white, mostly black into black, and mostly light blue into light blue; and preferably be able to adjust the scale it does this by so that I might prevent dust from becoming black spots and even maintain the greyscale range at the very edges of my lines?

It's not about turning the lines blue, it's just about isolating them from the rest of the drawing.
What you describe there,apart from return the lines to blue, is basically what I have been doing so far to isolate special portions of the drawing: copy out the lines and have a version with those extra lines erased.
I was hoping to find a better way to do that.

But what puts a greater challenge to me is to remove the blue lines from drawings where they were used for guides.

Perhaps I wasn't clear about this, but I am talking about modifying a color scan. I have a colored scan of black lines and light-blue lines on a white paper. And of course, each of those elements has a lot of noise. The black lines, because of errors in the scanner, have a slight tint on either side of them, about one pixel thin, one side red and the other side blue-green (I had not seen this particular distortion until now; I think I will switch my guide lines to yellow so I can't confuse them with the left edges of the black lines.) On top of that, those "black lines" now also have minor color fluctuations inside them. (This is hard to identify unless I zoom in about 800%)
On my greyscale scans I just used color curves to set all the black lines to a straight black and the white to a pure white, adjusting the middle ground to give me just as much fade between the two as I want (makes the lines smoother) but still keeping the solid parts solid.
I want to be able to do that with a color image, but the simple value-adjustment I had been making turns the edges into a colorful mulch of hippie dreams (which can be cured with desaturation) and the light-blue lines into a wide scale of value, as its natural value falls in the range I keep on the edges of the black lines.

I want to turn the grainy blue into a solid blue without warping the rest of the drawing.
This is just so that I can isolate them from the rest of the image; whether I delete them or shift them into a new layer for later evil purposes, I just need to isolate lines of a particular color from an image with very fine grain.